1. Field of the Invention
The present invention relates to a method for gain equalization, and a device and system for use in carrying out the method.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying signal light has been proposed or put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 xcexcm band or a 1.48 xcexcm band, a gain band including a wavelength band of 1.55 xcexcm can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain which is represented by a gain tilt or gain deviation. For example, in an EDFA, it is known that a gain tilt is produced at wavelengths in the vicinity of 1.55 xcexcm, and this gain tilt varies with total input power of signal light and pump light power to the EDFA.
A gain equalization method is known as measures against the wavelength characteristic of gain of an optical amplifier.
This method will be described with reference to FIGS. 1 to 4.
FIG. 1 is a block diagram showing a conventional optical communication system adopting WDM. A plurality of optical signals having different wavelengths are output from a plurality of optical senders (OS) 2(#1) to 2(#N), respectively, and next wavelength division multiplexed in an optical multiplexer 4 to obtain WDM signal light. The WDM signal light is next output to an optical transmission line 6. The optical transmission line 6 is configured by inserting a plurality of optical amplifiers 8 for compensating for losses and at least one gain equalizer 10 in an optical fiber transmission line 7. Each gain equalizer 10 may be provided by an optical filter. The WDM signal light transmitted by the optical transmission line 6 is separated into individual optical signals according to wavelengths by an optical demultiplexer 12, and these optical signals are next supplied to a plurality of optical receivers (OR) 14(#1) to 14(#N), respectively.
Referring to FIG. 2, there is shown an example of the spectrum of the WDM signal light output from the optical multiplexer 4 to the optical transmission line 6 in the system shown in FIG. 1. In FIG. 2, the vertical axis represents optical power, and the horizontal axis represents wavelength. In this example, the optical senders 2(#1) to 2(#N) output optical signals having wavelengths (xcex1) to (xcexN), respectively. When preemphasis is not considered, the optical powers of the optical signals in all the channels are equal to each other in general. In this example, the band of the WDM signal light is defined by the wavelength range of xcex1 to xcexN as shown by reference numeral 16.
If each optical amplifier 8 in the system shown in FIG. 1 has a wavelength characteristic of gain in the band 16 of the WDM signal light, a gain tilt or gain deviation is accumulated over the length of the optical transmission line 6, causing an interchannel deviation in signal power or signal-to-noise ratio (optical SNR). For example, in the case that each optical amplifier 8 has a wavelength characteristic of gain as shown in FIG. 3A, this wavelength characteristic of gain is accumulated to result in remarkable generation of a wavelength characteristic of total gain as shown in FIG. 3B.
In the gain equalization method, the wavelength characteristic of loss of each gain equalizer 10 is set so as to cancel the wavelength characteristic of total gain of the cascaded optical amplifiers 8.
This will now be described more specifically with reference to FIG. 4.
In FIG. 4, the broken line shown by reference numeral 18 represents the wavelength characteristic of total gain of the cascaded optical amplifiers 8, and the solid line shown by reference numeral 20 represents the wavelength characteristic of loss in the gain equalizer 10. In the example shown, the wavelength characteristic of total gain is canceled by the wavelength characteristic of loss in the band 16 of the WDM signal light, thereby achieving gain equalization in the whole of the optical transmission line 6.
In the case that an EDFA is used as each optical amplifier 8, the wavelength characteristic of gain of the EDFA is asymmetrical with respect to a wavelength axis in general. In contrast, the wavelength characteristic of loss of one optical filter usable as an element of each gain equalizer 10 is symmetrical with respect to a wavelength axis in general. Accordingly, in the case that each gain equalizer 10 includes only one optical filter, the asymmetrical wavelength characteristic of total gain of the cascaded optical amplifiers 8 cannot be compensated. As the optical filter, a dielectric multilayer filter, etalon filter, Mach-Zehnder filter, etc. are known. These filters can be precisely manufactured, and the reliability has been ensured.
As the related prior art to compensate for the asymmetrical wavelength characteristic of gain in an optical amplifier, it has been proposed to configure a gain equalizer by combining two or more optical filters having different wavelength characteristics of loss. With this configuration, the wavelength characteristic of gain can be canceled by the wavelength characteristic of loss with high accuracy in a given band of WDM signal light.
The wavelength characteristic of gain of an optical amplifier changes according to operating conditions such as a pumped condition of the optical amplifier and an input power of signal light. In a submarine optical repeater system, for example, there is a case that the input power to an optical amplifier may change because of an increase in optical fiber loss due to aging or because of cable patching for repairing. Such a change in system condition causes a change in operating conditions of the optical amplifier, resulting in a change in its wavelength characteristic of gain. Further, there is a possibility that the wavelength characteristic of gain may deviate from a design value because of variations in quality of optical amplifiers manufactured.
In the conventional gain equalization method using an optical filter having a fixed wavelength characteristic of loss, there arises a problem such that when the wavelength characteristic of gain of an optical amplifier changes from a characteristic shown by reference numeral 18 to a characteristic shown by reference numeral 18xe2x80x2 in FIG. 5 because of a change in system condition, the new wavelength characteristic of gain of the optical amplifier does not coincide with the wavelength characteristic of loss of the optical filter, causing an equalization error. The equalization error varies according to a system condition, and a large amount of variations in the equalization error may cause an interchannel deviation in signal power or optical SNR or may remarkably deteriorate a transmission quality in a certain channel.
From this point of view, there has been proposed a method using a variable gain equalizer having a variable wavelength characteristic of loss.
FIG. 6 is a block diagram showing a variable gain equalizer in the related art. An optical circulator 26 is provided between an input port 22 and an output port 24. A first port 30 of an AWG (arrayed waveguide) element 28 is connected to the optical circulator 26. An optical coupler 34, a variable optical attenuator 36, and a total reflector 38 are connected in this order to each of a plurality of second ports 32 of the AWG element 28. The power of reflected light in the total reflector 38 is detected by a photodetector 40 connected to the optical coupler 34, and the variable optical attenuator 36 is controlled so that a detection output from the photodetector 40 becomes constant. The first port 30 and each second port 32 of the AWG element 28 are coupled by a specific wavelength, so that a flat wavelength characteristic can be obtained by the configuration shown in FIG. 6.
However, the conventional variable gain equalizer shown in FIG. 6 requires expensive optical devices including the optical circulator 26 and the AWG element 28, and a plurality of (e.g., 32) feedback loops each including the variable optical attenuator 36 are also required, causing a complicated configuration and a high cost.
It is therefore an object of the present invention to provide a simple method for gain equalization which can suppress variations in equalization error due to variations in system condition.
It is another object of the present invention to provide a novel device (gain equalizer) having a variable wavelength characteristic of loss and a simple configuration.
It is a further object of the present invention to provide a system which can be used in carrying out the above method or which includes the above device.
In accordance with a first aspect of the present invention, there is provided a method for gain equalization. First, an optical transmission line including an optical amplifier for giving a gain to wavelength division multiplexed (WDM) signal light including an optical signal having a specific wavelength is provided (step (a)). Then, gain equalization is performed on the optical transmission line so that a gain changing substantially monotonously with respect to wavelength is obtained (step (b)). Further, gain equalization is performed on the optical transmission line so that a gain substantially fixed with respect to wavelength is obtained (step (c)). The order of execution of the step (b) and the step (c) is arbitrary.
For example, by executing the step (c) after the step (b), gain equalization can be simply performed on the gain changing substantially monotonously, so that variations in equalization error due to variations in system condition can be easily suppressed.
In accordance with a second aspect of the present invention, there is provided a device comprising an optical element provided on an optical path of wavelength division multiplexed (WDM) signal light including an optical signal having a specific wavelength, for giving a loss changing with a periodicity with respect to wavelength to said WDM signal light; and means acting on said optical element so that said periodicity is changed. The specific wavelength is substantially coincident with a wavelength giving a minimum or maximum of the loss in said optical element, for example. This device is suitable for gain equalization on the gain changing substantially monotonously with respect to wavelength, so that this device can be used in carrying out the method according to the first aspect of the present invention.
In accordance with a third aspect of the present invention, there is provided a system comprising an optical fiber transmission line for transmitting wavelength division multiplexed (WDM) signal light including an optical signal having a specific wavelength; an optical amplifier optically connected to said optical fiber transmission line; and an active gain equalizer optically connected to said optical fiber transmission line. The active gain equalizer may be provided by the device according to the second aspect of the present invention.
In accordance with a fourth aspect of the present invention, there is provided a system comprising an optical fiber span consisting of a plurality of sections each comprising an optical amplifier and an optical fiber transmission line for transmitting wavelength division multiplexed (WDM) signal light including an optical signal having a specific wavelength. Each section further comprises a passive gain equalizer having a fixed wavelength characteristic of gain or loss, and an active gain equalizer having a variable wavelength characteristic of loss. The active gain equalizer may be provided by the device according to the second aspect of the present invention.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.